In recent years, 100 Gigabit-per-second (Gbs) long-distance optical transmission has been implemented by dual-polarization quadrature phase-shift keying (DP-QPSK) using a digital coherent technology. To further improve transmission capacity, greater-level modulation schemes such as polarization division multiplexed 16 quadrature amplitude modulation (16-QAM) are being developed. Nyquist wavelength division multiplexing (WDM) is also being developed to increase the transmission rate, which technique transmits data on more wavelength channels by narrowing the channel spacing using a square-shaped transmission spectrum.
Meanwhile, demand for reducing the size of optical transceiver, which is used as a frontend module of optical transmission systems, is increasing. At present, Lithium niobate (LiNbO3) Mach-Zehnder interferometer is typically used as an electro-optic modulator. In order to reduce a device size, it is desired to actualize multilevel modulation (e.g., DP-QPSK, DP-16QAM, etc.) using a semiconductor Mach-Zehnder interferometer.
There is an intrinsic problem in semiconductor optical modulators that the modulation characteristic (i.e., the relationship between applied voltage and optical phase change) varies depending on the wavelength of light input to the modulator. In semiconductor optical modulators, the absorption edge wavelength of the semiconductor material changes according to applied voltage, and the phase of light is modulated making use of the phase shift due to absorption based on Kramers-Kronig relations. Hence, semiconductor optical modulators have wavelength dependency such that the closer to the absorption-edge-wavelength the light to be modulated is, the greater the phase change with respect to the voltage change.
To address the wavelength dependency of the modulation characteristic of semiconductor optical modulators, several techniques for controlling a substrate bias voltage or amplitude of a drive signal according to the wavelength of input light are proposed. The first technique is to set the substrate bias voltage to a predetermined level according to the wavelength, and drive the modulator at a constant amplitude of a drive signal regardless of the wavelength. See, for example, Japanese Laid-open Patent Publication No. 2005-326548 A.
The second technique is to perform feedback control on the substrate bias voltage or drive signal amplitude. A low frequency signal is superimposed on driving data signals, and output light signals are monitored. Responsive to the monitoring result, the substrate bias voltage and/or the amplitude of the modulator drive signal is controlled. See, for example, Japanese Laid-open patent publication No. 2012-257164 A.
In some modulation schemes, the modulation index or the modulation depth of an optical modulator needs to be set to an arbitrary level at or under 100%. However, semiconductor Mach-Zehnder modulators have a problem that the voltage-to-phase characteristic, or the wavelength characteristic, or the driving amplitude varies between the waveguide pair of the Mach-Zehnder interferometer. The voltage-to-phase characteristic may also change differently with time between the two waveguides.
It is desired for fiber optic communication systems to control the modulation index to an arbitrary level and maintain the optimum condition for modulation even if characteristics fluctuate or change differently with time between the two waveguides of a Mach-Zehnder interferometer.